System and method for verifying digital signatures on certificates

ABSTRACT

A system and method for verifying a digital signature on a certificate, which may be used in the processing of encoded messages. In one embodiment, when a digital signature is successfully verified in a signature verification operation, the public key used to verify that digital signature is cached. When a subsequent attempt to verify the digital signature is made, the public key to be used to verify the digital signature is compared to the cached key. If the keys match, the digital signature can be successfully verified without requiring that a signature verification operation in which some data is decoded using the public key be performed.

CROSS REFERENCE TO RELATED APPLICATION

This application is a continuation of prior U.S. patent application Ser.No. 12/771,194, filed on Apr. 30, 2010, which is a continuation of U.S.patent application Ser. No. 10/975,988, filed on Oct. 29, 2004. U.S.patent application Ser. No. 10/975,988 issued to U.S. Pat. No. 7,716,139on May 11, 2010. The entire contents of application Ser. No. 12/771,194and of application Ser. No. 10/975,988 are hereby incorporated byreference.

FIELD OF THE INVENTION

The invention relates generally to the processing of messages, such ase-mail messages, and more specifically to a system and method forvalidating certificates used in the processing of encoded messages.

BACKGROUND OF THE INVENTION

Electronic mail (“e-mail”) messages may be encoded using one of a numberof known protocols. Some of these protocols, such as Secure MultipleInternet Mail Extensions (“S/MIME”) for example, rely on public andprivate encryption keys to provide confidentiality and integrity, and ona Public Key Infrastructure (PKI) to communicate information thatprovides authentication and authorization. Data encrypted using aprivate key of a private key/public key pair can only be decrypted usingthe corresponding public key of the pair, and vice-versa. Theauthenticity of public keys used in the encoding of messages isvalidated using certificates. In particular, if a user of a computingdevice wishes to encrypt a message before the message is sent to aparticular individual, the user will require a certificate for thatindividual. That certificate will typically comprise the public key ofthe individual, as well as other identification-related information.

Certificates are digital documents that are typically issued bycertification authorities. In order to trust a particular public key,the public key typically needs to be issued by a certification authoritythat is also trusted, or by an entity associated with the trustedcertification authority. The relationship between a trustedcertification authority and an issued public key can be represented by aseries of related certificates, also referred to as a certificate chain.The certificate chain can be followed to determine the validity of acertificate.

Typically, a certification authority will digitally sign eachcertificate that it issues, to certify that a specific public keybelongs to the purported owner as indicated on the respectivecertificate. In building certificate chains, the digital signatures onthe certificates of the chain often need to be verified. Verification ofa digital signature on a certificate is a process that requires thepublic key of the certification authority that issued the certificate.

SUMMARY OF THE INVENTION

The verification process can be time-consuming and costly (e.g. in termsof computing resource usage), particularly where the verifications areperformed on smaller devices, such as mobile devices for example. Wheremultiple certificates are processed on a user's computing device, thesame digital signature may be subject to verification more than once.Embodiments of the invention are generally directed to a system andmethod that facilitates more efficient verification of digitalsignatures on certificates by storing certain information employed insignature verification operations for reuse.

In a broad aspect of the invention, there is provided a method ofverifying a digital signature on a certificate on a computing device,the method comprising the steps of: performing a first signatureverification operation on the digital signature using a first public keyassociated with an issuer of the certificate; determining if the digitalsignature is successfully verified in the first signature verificationoperation; storing the first public key in a memory store; receiving arequest to perform a second signature verification operation on thedigital signature using a second public key associated with an issuer ofthe certificate; comparing the second public key with the first publickey stored in the memory store to determine if the first and secondpublic keys match; and indicating successful verification of the digitalsignature in response to the request if the digital signature wassuccessfully verified in the first signature verification operation andif a match is determined at the comparing step, whereby the secondsignature verification operation need not be performed.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of embodiments of the invention, and to showmore clearly how it may be carried into effect, reference will now bemade, by way of example, to the accompanying drawings in which:

FIG. 1 is a block diagram of a mobile device in one exampleimplementation;

FIG. 2 is a block diagram of a communication subsystem component of themobile device of FIG. 1;

FIG. 3 is a block diagram of a node of a wireless network;

FIG. 4 is a block diagram illustrating components of a host system inone example configuration;

FIG. 5 is a block diagram showing an example of a certificate chain;

FIG. 6 is a block diagram illustrating components of an example of anencoded message;

FIG. 7A is a block diagram showing two example certificate chains;

FIG. 7B is a block diagram showing cross-certificates linking thecertificate chains of FIG. 7A;

FIG. 8A is a flowchart illustrating steps in a method of verifying adigital signature on a certificate in an embodiment of the invention;and

FIG. 8B is a flowchart illustrating steps in a method of verifying adigital signature on a certificate in another embodiment of theinvention.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE INVENTION

Some embodiments of the invention make use of a mobile station. A mobilestation is a two-way communication device with advanced datacommunication capabilities having the capability to communicate withother computer systems, and is also referred to herein generally as amobile device. A mobile device may also include the capability for voicecommunications. Depending on the functionality provided by a mobiledevice, it may be referred to as a data messaging device, a two-waypager, a cellular telephone with data messaging capabilities, a wirelessInternet appliance, or a data communication device (with or withouttelephony capabilities). A mobile device communicates with other devicesthrough a network of transceiver stations.

To aid the reader in understanding the structure of a mobile device andhow it communicates with other devices, reference is made to FIGS. 1through 3.

Referring first to FIG. 1, a block diagram of a mobile device in oneexample implementation is shown generally as 100. Mobile device 100comprises a number of components, the controlling component beingmicroprocessor 102. Microprocessor 102 controls the overall operation ofmobile device 100. Communication functions, including data and voicecommunications, are performed through communication subsystem 104.Communication subsystem 104 receives messages from and sends messages toa wireless network 200. In this example implementation of mobile device100, communication subsystem 104 is configured in accordance with theGlobal System for Mobile Communication (GSM) and General Packet RadioServices (GPRS) standards. The GSM/GPRS wireless network is usedworldwide and it is expected that these standards will be supersededeventually by Enhanced Data GSM Environment (EDGE) and Universal MobileTelecommunications Service (UMTS). New standards are still beingdefined, but it is believed that they will have similarities to thenetwork behaviour described herein, and it will also be understood bypersons skilled in the art that the invention is intended to use anyother suitable standards that are developed in the future. The wirelesslink connecting communication subsystem 104 with network 200 representsone or more different Radio Frequency (RF) channels, operating accordingto defined protocols specified for GSM/GPRS communications. With newernetwork protocols, these channels are capable of supporting both circuitswitched voice communications and packet switched data communications.

Although the wireless network associated with mobile device 100 is aGSM/GPRS wireless network in one example implementation of mobile device100, other wireless networks may also be associated with mobile device100 in variant implementations. Different types of wireless networksthat may be employed include, for example, data-centric wirelessnetworks, voice-centric wireless networks, and dual-mode networks thatcan support both voice and data communications over the same physicalbase stations. Combined dual-mode networks include, but are not limitedto, Code Division Multiple Access (CDMA) or CDMA2000 networks, GSM/GPRSnetworks (as mentioned above), and future third-generation (3G) networkslike EDGE and UMTS. Some older examples of data-centric networks includethe Mobitex™ Radio Network and the DataTAC™ Radio Network. Examples ofolder voice-centric data networks include Personal Communication Systems(PCS) networks like GSM and Time Division Multiple Access (TDMA)systems.

Microprocessor 102 also interacts with additional subsystems such as aRandom Access Memory (RAM) 106, flash memory 108, display 110, auxiliaryinput/output (I/O) subsystem 112, serial port 114, keyboard 116, speaker118, microphone 120, short-range communications 122 and other devices124.

Some of the subsystems of mobile device 100 performcommunication-related functions, whereas other subsystems may provide“resident” or on-device functions. By way of example, display 110 andkeyboard 116 may be used for both communication-related functions, suchas entering a text message for transmission over network 200, anddevice-resident functions such as a calculator or task list. Operatingsystem software used by microprocessor 102 is typically stored in apersistent store such as flash memory 108, which may alternatively be aread-only memory (ROM) or similar storage element (not shown). Thoseskilled in the art will appreciate that the operating system, specificdevice applications, or parts thereof, may be temporarily loaded into avolatile store such as RAM 106.

Mobile device 100 may send and receive communication signals overnetwork 200 after required network registration or activation procedureshave been completed. Network access is associated with a subscriber oruser of a mobile device 100. To identify a subscriber, mobile device 100requires a Subscriber Identity Module or “SIM” card 126 to be insertedin a SIM interface 128 in order to communicate with a network. SIM 126is one type of a conventional “smart card” used to identify a subscriberof mobile device 100 and to personalize the mobile device 100, amongother things. Without SIM 126, mobile device 100 is not fullyoperational for communication with network 200. By inserting SIM 126into SIM interface 128, a subscriber can access all subscribed services.Services could include: web browsing and messaging such as e-mail, voicemail, Short Message Service (SMS), and Multimedia Messaging Services(MMS). More advanced services may include: point of sale, field serviceand sales force automation. SIM 126 includes a processor and memory forstoring information. Once SIM 126 is inserted in SIM interface 128, itis coupled to microprocessor 102. In order to identify the subscriber,SIM 126 contains some user parameters such as an International MobileSubscriber Identity (IMSI). An advantage of using SIM 126 is that asubscriber is not necessarily bound by any single physical mobiledevice. SIM 126 may store additional subscriber information for a mobiledevice as well, including datebook (or calendar) information and recentcall information.

Mobile device 100 is a battery-powered device and includes a batteryinterface 132 for receiving one or more rechargeable batteries 130.Battery interface 132 is coupled to a regulator (not shown), whichassists battery 130 in providing power V+ to mobile device 100. Althoughcurrent technology makes use of a battery, future technologies such asmicro fuel cells may provide the power to mobile device 100.

Microprocessor 102, in addition to its operating system functions,enables execution of software applications on mobile device 100. A setof applications that control basic device operations, including data andvoice communication applications, will normally be installed on mobiledevice 100 during its manufacture. Another application that may beloaded onto mobile device 100 would be a personal information manager(PIM). A PIM has functionality to organize and manage data items ofinterest to a subscriber, such as, but not limited to, e-mail, calendarevents, voice mails, appointments, and task items. A PIM application hasthe ability to send and receive data items via wireless network 200. PIMdata items may be seamlessly integrated, synchronized, and updated viawireless network 200 with the mobile device subscriber's correspondingdata items stored and/or associated with a host computer system. Thisfunctionality creates a mirrored host computer on mobile device 100 withrespect to such items. This can be particularly advantageous where thehost computer system is the mobile device subscriber's office computersystem.

Additional applications may also be loaded onto mobile device 100through network 200, auxiliary I/O subsystem 112, serial port 114,short-range communications subsystem 122, or any other suitablesubsystem 124. This flexibility in application installation increasesthe functionality of mobile device 100 and may provide enhancedon-device functions, communication-related functions, or both. Forexample, secure communication applications may enable electroniccommerce functions and other such financial transactions to be performedusing mobile device 100.

Serial port 114 enables a subscriber to set preferences through anexternal device or software application and extends the capabilities ofmobile device 100 by providing for information or software downloads tomobile device 100 other than through a wireless communication network.The alternate download path may, for example, be used to load anencryption key onto mobile device 100 through a direct and thus reliableand trusted connection to provide secure device communication.

Short-range communications subsystem 122 provides for communicationbetween mobile device 100 and different systems or devices, without theuse of network 200. For example, subsystem 122 may include an infrareddevice and associated circuits and components for short-rangecommunication. Examples of short range communication would includestandards developed by the Infrared Data Association (IrDA), Bluetooth,and the 802.11 family of standards developed by IEEE.

In use, a received signal such as a text message, an e-mail message, orweb page download will be processed by communication subsystem 104 andinput to microprocessor 102. Microprocessor 102 will then process thereceived signal for output to display 110 or alternatively to auxiliaryI/O subsystem 112. A subscriber may also compose data items, such ase-mail messages, for example, using keyboard 116 in conjunction withdisplay 110 and possibly auxiliary I/O subsystem 112. Auxiliarysubsystem 112 may include devices such as: a touch screen, mouse, trackball, infrared fingerprint detector, or a roller wheel with dynamicbutton pressing capability. Keyboard 116 is an alphanumeric keyboardand/or telephone-type keypad. A composed item may be transmitted overnetwork 200 through communication subsystem 104.

For voice communications, the overall operation of mobile device 100 issubstantially similar, except that the received signals would be outputto speaker 118, and signals for transmission would be generated bymicrophone 120. Alternative voice or audio I/O subsystems, such as avoice message recording subsystem, may also be implemented on mobiledevice 100. Although voice or audio signal output is accomplishedprimarily through speaker 118, display 110 may also be used to provideadditional information such as the identity of a calling party, durationof a voice call, or other voice call related information.

Referring now to FIG. 2, a block diagram of the communication subsystemcomponent 104 of FIG. 1 is shown. Communication subsystem 104 comprisesa receiver 150, a transmitter 152, one or more embedded or internalantenna elements 154, 156, Local Oscillators (LOs) 158, and a processingmodule such as a Digital Signal Processor (DSP) 160.

The particular design of communication subsystem 104 is dependent uponthe network 200 in which mobile device 100 is intended to operate, thusit should be understood that the design illustrated in FIG. 2 servesonly as one example. Signals received by antenna 154 through network 200are input to receiver 150, which may perform such common receiverfunctions as signal amplification, frequency down conversion, filtering,channel selection, and analog-to-digital (A/D) conversion. A/Dconversion of a received signal allows more complex communicationfunctions such as demodulation and decoding to be performed in DSP 160.In a similar manner, signals to be transmitted are processed, includingmodulation and encoding, by DSP 160. These DSP-processed signals areinput to transmitter 152 for digital-to-analog (D/A) conversion,frequency up conversion, filtering, amplification and transmission overnetwork 200 via antenna 156. DSP 160 not only processes communicationsignals, but also provides for receiver and transmitter control. Forexample, the gains applied to communication signals in receiver 150 andtransmitter 152 may be adaptively controlled through automatic gaincontrol algorithms implemented in DSP 160.

The wireless link between mobile device 100 and a network 200 maycontain one or more different channels, typically different RF channels,and associated protocols used between mobile device 100 and network 200.A RF channel is a limited resource that must be conserved, typically dueto limits in overall bandwidth and limited battery power of mobiledevice 100.

When mobile device 100 is fully operational, transmitter 152 istypically keyed or turned on only when it is sending to network 200 andis otherwise turned off to conserve resources. Similarly, receiver 150is periodically turned off to conserve power until it is needed toreceive signals or information (if at all) during designated timeperiods.

Referring now to FIG. 3, a block diagram of a node of a wireless networkis shown as 202. In practice, network 200 comprises one or more nodes202. Mobile device 100 communicates with a node 202 within wirelessnetwork 200. In the example implementation of FIG. 3, node 202 isconfigured in accordance with General Packet Radio Service (GPRS) andGlobal Systems for Mobile (GSM) technologies. Node 202 includes a basestation controller (BSC) 204 with an associated tower station 206, aPacket Control Unit (PCU) 208 added for GPRS support in GSM, a MobileSwitching Center (MSC) 210, a Home Location Register (HLR) 212, aVisitor Location Registry (VLR) 214, a Serving GPRS Support Node (SGSN)216, a Gateway GPRS Support Node (GGSN) 218, and a Dynamic HostConfiguration Protocol (DHCP) 220. This list of components is not meantto be an exhaustive list of the components of every node 202 within aGSM/GPRS network, but rather a list of components that are commonly usedin communications through network 200.

In a GSM network, MSC 210 is coupled to BSC 204 and to a landlinenetwork, such as a Public Switched Telephone Network (PSTN) 222 tosatisfy circuit switched requirements. The connection through PCU 208,SGSN 216 and GGSN 218 to the public or private network (Internet) 224(also referred to herein generally as a shared network infrastructure)represents the data path for GPRS capable mobile devices. In a GSMnetwork extended with GPRS capabilities, BSC 204 also contains a PacketControl Unit (PCU) 208 that connects to SGSN 216 to controlsegmentation, radio channel allocation and to satisfy packet switchedrequirements. To track mobile device location and availability for bothcircuit switched and packet switched management, HLR 212 is sharedbetween MSC 210 and SGSN 216. Access to VLR 214 is controlled by MSC210.

Station 206 is a fixed transceiver station. Station 206 and BSC 204together form the fixed transceiver equipment. The fixed transceiverequipment provides wireless network coverage for a particular coveragearea commonly referred to as a “cell”. The fixed transceiver equipmenttransmits communication signals to and receives communication signalsfrom mobile devices within its cell via station 206. The fixedtransceiver equipment normally performs such functions as modulation andpossibly encoding and/or encryption of signals to be transmitted to themobile device in accordance with particular, usually predetermined,communication protocols and parameters, under control of its controller.The fixed transceiver equipment similarly demodulates and possiblydecodes and decrypts, if necessary, any communication signals receivedfrom mobile device 100 within its cell. Communication protocols andparameters may vary between different nodes. For example, one node mayemploy a different modulation scheme and operate at differentfrequencies than other nodes.

For all mobile devices 100 registered with a specific network, permanentconfiguration data such as a user profile is stored in HLR 212. HLR 212also contains location information for each registered mobile device andcan be queried to determine the current location of a mobile device. MSC210 is responsible for a group of location areas and stores the data ofthe mobile devices currently in its area of responsibility in VLR 214.Further VLR 214 also contains information on mobile devices that arevisiting other networks. The information in VLR 214 includes part of thepermanent mobile device data transmitted from HLR 212 to VLR 214 forfaster access. By moving additional information from a remote HLR 212node to VLR 214, the amount of traffic between these nodes can bereduced so that voice and data services can be provided with fasterresponse times and at the same time requiring less use of computingresources.

SGSN 216 and GGSN 218 are elements added for GPRS support; namely packetswitched data support, within GSM. SGSN 216 and MSC 210 have similarresponsibilities within wireless network 200 by keeping track of thelocation of each mobile device 100. SGSN 216 also performs securityfunctions and access control for data traffic on network 200. GGSN 218provides internetworking connections with external packet switchednetworks and connects to one or more SGSN's 216 via an Internet Protocol(IP) backbone network operated within the network 200. During normaloperations, a given mobile device 100 must perform a “GPRS Attach” toacquire an IP address and to access data services. This requirement isnot present in circuit switched voice channels as Integrated ServicesDigital Network (ISDN) addresses are used for routing incoming andoutgoing calls. Currently, all GPRS capable networks use private,dynamically assigned IP addresses, thus requiring a DHCP server 220connected to the GGSN 218. There are many mechanisms for dynamic IPassignment, including using a combination of a Remote AuthenticationDial-In User Service (RADIUS) server and DHCP server. Once the GPRSAttach is complete, a logical connection is established from a mobiledevice 100, through PCU 208, and SGSN 216 to an Access Point Node (APN)within GGSN 218. The APN represents a logical end of an IP tunnel thatcan either access direct Internet compatible services or private networkconnections. The APN also represents a security mechanism for network200, insofar as each mobile device 100 must be assigned to one or moreAPNs and mobile devices 100 cannot exchange data without firstperforming a GPRS Attach to an APN that it has been authorized to use.The APN may be considered to be similar to an Internet domain name suchas “myconnection.wireless.com”.

Once the GPRS Attach is complete, a tunnel is created and all traffic isexchanged within standard IP packets using any protocol that can besupported in IP packets. This includes tunneling methods such as IP overIP as in the case with some IPSecurity (IPsec) connections used withVirtual Private Networks (VPN). These tunnels are also referred to asPacket Data Protocol (PDP) Contexts and there are a limited number ofthese available in the network 200. To maximize use of the PDP Contexts,network 200 will run an idle timer for each PDP Context to determine ifthere is a lack of activity. When a mobile device 100 is not using itsPDP Context, the PDP Context can be deallocated and the IP addressreturned to the IP address pool managed by DHCP server 220.

Referring now to FIG. 4, a block diagram illustrating components of ahost system in one example configuration is shown. Host system 250 willtypically be a corporate office or other local area network (LAN), butmay instead be a home office computer or some other private system, forexample, in variant implementations. In this example shown in FIG. 4,host system 250 is depicted as a LAN of an organization to which a userof mobile device 100 belongs.

LAN 250 comprises a number of network components connected to each otherby LAN connections 260. For instance, a user's desktop computer 262 awith an accompanying cradle 264 for the user's mobile device 100 issituated on LAN 250. Cradle 264 for mobile device 100 may be coupled tocomputer 262 a by a serial or a Universal Serial Bus (USB) connection,for example. Other user computers 262 b are also situated on LAN 250,and each may or may not be equipped with an accompanying cradle 264 fora mobile device. Cradle 264 facilitates the loading of information (e.g.PIM data, private symmetric encryption keys to facilitate securecommunications between mobile device 100 and LAN 250) from user computer262 a to mobile device 100, and may be particularly useful for bulkinformation updates often performed in initializing mobile device 100for use. The information downloaded to mobile device 100 may includecertificates used in the exchange of messages. It will be understood bypersons skilled in the art that user computers 262 a, 262 b willtypically be also connected to other peripheral devices not explicitlyshown in FIG. 4.

Furthermore, only a subset of network components of LAN 250 are shown inFIG. 4 for ease of exposition, and it will be understood by personsskilled in the art that LAN 250 will comprise additional components notexplicitly shown in FIG. 4, for this example configuration. Moregenerally, LAN 250 may represent a smaller part of a larger network [notshown] of the organization, and may comprise different components and/orbe arranged in different topologies than that shown in the example ofFIG. 4.

In this example, mobile device 100 communicates with LAN 250 through anode 202 of wireless network 200 and a shared network infrastructure 224such as a service provider network or the public Internet. Access to LAN250 may be provided through one or more routers [not shown], andcomputing devices of LAN 250 may operate from behind a firewall or proxyserver 266.

In a variant implementation, LAN 250 comprises a wireless VPN router[not shown] to facilitate data exchange between the LAN 250 and mobiledevice 100. The concept of a wireless VPN router is new in the wirelessindustry and implies that a VPN connection can be established directlythrough a specific wireless network to mobile device 100. Thepossibility of using a wireless VPN router has only recently beenavailable and could be used when the new Internet Protocol (IP) Version6 (IPV6) arrives into IP-based wireless networks. This new protocol willprovide enough IP addresses to dedicate an IP address to every mobiledevice, making it possible to push information to a mobile device at anytime. An advantage of using a wireless VPN router is that it could be anoff-the-shelf VPN component, not requiring a separate wireless gatewayand separate wireless infrastructure to be used. A VPN connection wouldpreferably be a Transmission Control Protocol (TCP)/IP or User DatagramProtocol (UDP)/IP connection to deliver the messages directly to mobiledevice 100 in this variant implementation.

Messages intended for a user of mobile device 100 are initially receivedby a message server 268 of LAN 250. Such messages may originate from anyof a number of sources. For instance, a message may have been sent by asender from a computer 262 b within LAN 250, from a different mobiledevice [not shown] connected to wireless network 200 or to a differentwireless network, or from a different computing device or other devicecapable of sending messages, via the shared network infrastructure 224,and possibly through an application service provider (ASP) or Internetservice provider (ISP), for example.

Message server 268 typically acts as the primary interface for theexchange of messages, particularly e-mail messages, within theorganization and over the shared network infrastructure 224. Each userin the organization that has been set up to send and receive messages istypically associated with a user account managed by message server 268.One example of a message server 268 is a Microsoft Exchange™ Server. Insome implementations, LAN 250 may comprise multiple message servers 268.Message server 268 may also be adapted to provide additional functionsbeyond message management, including the management of data associatedwith calendars and task lists, for example.

When messages are received by message server 268, they are typicallystored in a message store [not explicitly shown], from which messagescan be subsequently retrieved and delivered to users. For instance, ane-mail client application operating on a user's computer 262 a mayrequest the e-mail messages associated with that user's account storedon message server 268. These messages would then typically be retrievedfrom message server 268 and stored locally on computer 262 a.

When operating mobile device 100, the user may wish to have e-mailmessages retrieved for delivery to the handheld. An e-mail clientapplication operating on mobile device 100 may also request messagesassociated with the user's account from message server 268. The e-mailclient may be configured (either by the user or by an administrator,possibly in accordance with an organization's information technology(IT) policy) to make this request at the direction of the user, at somepre-defined time interval, or upon the occurrence of some pre-definedevent. In some implementations, mobile device 100 is assigned its owne-mail address, and messages addressed specifically to mobile device 100are automatically redirected to mobile device 100 as they are receivedby message server 268.

To facilitate the wireless communication of messages and message-relateddata between mobile device 100 and components of LAN 250, a number ofwireless communications support components 270 may be provided. In thisexample implementation, wireless communications support components 270comprise a message management server 272, for example. Messagemanagement server 272 is used to specifically provide support for themanagement of messages, such as e-mail messages, that are to be handledby mobile devices. Generally, while messages are still stored on messageserver 268, message management server 272 can be used to control when,if, and how messages should be sent to mobile device 100. Messagemanagement server 272 also facilitates the handling of messages composedon mobile device 100, which are sent to message server 268 forsubsequent delivery.

For example, message management server 272 may: monitor the user's“mailbox” (e.g. the message store associated with the user's account onmessage server 268) for new e-mail messages; apply user-definablefilters to new messages to determine if and how the messages will berelayed to the user's mobile device 100; compress and encrypt newmessages (e.g. using an encryption technique such as Data EncryptionStandard (DES) or Triple DES) and push them to mobile device 100 via theshared network infrastructure 224 and wireless network 200; and receivemessages composed on mobile device 100 (e.g. encrypted using TripleDES), decrypt and decompress the composed messages, re-format thecomposed messages if desired so that they will appear to have originatedfrom the user's computer 262 a, and re-route the composed messages tomessage server 268 for delivery.

Certain properties or restrictions associated with messages that are tobe sent from and/or received by mobile device 100 can be defined (e.g.by an administrator in accordance with IT policy) and enforced bymessage management server 272. These may include whether mobile device100 may receive encrypted and/or signed messages, minimum encryption keysizes, whether outgoing messages must be encrypted and/or signed, andwhether copies of all secure messages sent from mobile device 100 are tobe sent to a pre-defined copy address, for example.

Message management server 272 may also be adapted to provide othercontrol functions, such as only pushing certain message information orpre-defined portions (e.g. “blocks”) of a message stored on messageserver 268 to mobile device 100. For example, when a message isinitially retrieved by mobile device 100 from message server 268,message management server 272 is adapted to push only the first part ofa message to mobile device 100, with the part being of a pre-definedsize (e.g. 2 KB). The user can then request more of the message, to bedelivered in similar-sized blocks by message management server 272 tomobile device 100, possibly up to a maximum pre-defined message size.

Accordingly, message management server 272 facilitates better controlover the type of data and the amount of data that is communicated tomobile device 100, and can help to minimize potential waste of bandwidthor other resources.

It will be understood by persons skilled in the art that messagemanagement server 272 need not be implemented on a separate physicalserver in LAN 250 or other network. For example, some or all of thefunctions associated with message management server 272 may beintegrated with message server 268, or some other server in LAN 250.Furthermore, LAN 250 may comprise multiple message management servers272, particularly in variant implementations where a large number ofmobile devices needs to be supported.

Embodiments of the invention relate generally to certificates used inthe processing of encoded messages, such as e-mail messages that areencrypted and/or signed. While Simple Mail Transfer Protocol (SMTP),RFC822 headers, and Multipurpose Internet Mail Extensions (MIME) bodyparts may be used to define the format of a typical e-mail message notrequiring encoding, Secure/MIME (S/MIME), a version of the MIMEprotocol, may be used in the communication of encoded messages (i.e. insecure messaging applications). S/MIME enables end-to-end authenticationand confidentiality, and protects data integrity and privacy from thetime an originator of a message sends a message until it is decoded andread by the message recipient. Other known standards and protocols maybe employed to facilitate secure message communication, such as PrettyGood Privacy™ (PGP), OpenPGP, and others known in the art.

Secure messaging protocols such as S/MIME rely on public and privateencryption keys to provide confidentiality and integrity, and on aPublic Key Infrastructure (PKI) to communicate information that providesauthentication and authorization. Data encrypted using a private key ofa private key/public key pair can only be decrypted using thecorresponding public key of the pair, and vice-versa. Private keyinformation is never made public, whereas public key information isshared.

For example, if a sender wishes to send a message to a recipient inencrypted form, the recipient's public key is used to encrypt a message,which can then be decrypted only using the recipient's private key.Alternatively, in some encoding techniques, a one-time session key isgenerated and used to encrypt the body of a message, typically with asymmetric encryption technique (e.g. Triple DES). The session key isthen encrypted using the recipient's public key (e.g. with a public keyencryption algorithm such as RSA), which can then be decrypted onlyusing the recipient's private key. The decrypted session key can then beused to decrypt the message body. The message header may be used tospecify the particular encryption scheme that must be used to decryptthe message. Other encryption techniques based on public keycryptography may be used in variant implementations. However, in each ofthese cases, only the recipient's private key may be used to facilitatedecryption of the message, and in this way, the confidentiality ofmessages can be maintained.

As a further example, a sender may sign a message using a digitalsignature. A digital signature is a digest of the message (e.g. a hashof the message) encoded using the sender's private key, which can thenbe appended to the outgoing message. To verify the digital signature ofthe message when received, the recipient uses the same technique as thesender (e.g. using the same standard hash algorithm) to obtain a digestof the received message. The recipient also uses the sender's public keyto decode the digital signature, in order to obtain what should be amatching digest for the received message. If the digests of the receivedmessage do not match, this suggests that either the message content waschanged during transport and/or the message did not originate from thesender whose public key was used for verification. Digital signaturealgorithms are designed in such a way that only someone with knowledgeof the sender's private key should be able to encode a signature thatthe recipient will decode correctly using the sender's public key.Therefore, by verifying a digital signature in this way, authenticationof the sender and message integrity can be maintained.

An encoded message may be encrypted, signed, or both encrypted andsigned. The authenticity of public keys used in these operations isvalidated using certificates. A certificate is a digital document issuedby a certificate authority (CA). Certificates are used to authenticatethe association between users and their public keys, and essentially,provides a level of trust in the authenticity of the users' public keys.Certificates contain information about the certificate holder, withcertificate contents typically formatted in accordance with an acceptedstandard (e.g. X.509).

Consider FIG. 5, in which an example certificate chain 300 is shown.Certificate 310 issued to “John Smith” is an example of a certificateissued to an individual, which may be referred to as an end entitycertificate. End entity certificate 310 typically identifies thecertificate holder 312 (i.e. John Smith in this example) and the issuerof the certificate 314, and includes a digital signature of the issuer316 and the certificate holder's public key 318. Certificate 310 willalso typically include other information and attributes that identifythe certificate holder (e.g. e-mail address, organization name,organizational unit name, location, etc.). When the individual composesa message to be sent to a recipient, it is customary to include thatindividual's certificate 310 with the message.

For a public key to be trusted, its issuing organization must betrusted. The relationship between a trusted CA and a user's public keycan be represented by a series of related certificates, also referred toas a certificate chain. The certificate chain can be followed todetermine the validity of a certificate.

For instance, in the example certificate chain 300 shown in FIG. 5, therecipient of a message purported to be sent by John Smith may wish toverify the trust status of certificate 310 attached to the receivedmessage. To verify the trust status of certificate 310 on a recipient'scomputing device (e.g. computer 262 a of FIG. 4) for example, thecertificate 320 of issuer ABC is obtained, and used to verify thatcertificate 310 was indeed signed by issuer ABC. Certificate 320 mayalready be stored in a certificate store on the computing device, or itmay need to be retrieved from a certificate source (e.g. LDAP server 284of FIG. 4 or some other public or private LDAP server). If certificate320 is already stored in the recipient's computing device and thecertificate has been designated as trusted by the recipient, thencertificate 310 is considered to be trusted since it chains to a stored,trusted certificate.

However, in the example shown in FIG. 5, certificate 330 is alsorequired to verify the trust status of certificate 310. Certificate 330is self-signed, and is referred to as a “root certificate”. Accordingly,certificate 320 may be referred to as an “intermediate certificate” incertificate chain 300; any given certificate chain to a rootcertificate, assuming a chain to the root certificate can be determinedfor a particular end entity certificate, may contain zero, one, ormultiple intermediate certificates. If certificate 330 is a rootcertificate issued by a trusted source (from a large certificateauthority such as Verisign or Entrust, for example), then certificate310 may be considered to be trusted since it chains to a trustedcertificate. The implication is that both the sender and the recipientof the message trust the source of the root certificate 330. If acertificate cannot be chained to a trusted certificate, the certificatemay be considered to be “not trusted”.

Certificate servers store information about certificates and listsidentifying certificates that have been revoked. These certificateservers can be accessed to obtain certificates and to verify certificateauthenticity and revocation status. For example, a Lightweight DirectoryAccess Protocol (LDAP) server may be used to obtain certificates, and anOnline Certificate Status Protocol (OCSP) server may be used to verifycertificate revocation status.

Standard e-mail security protocols typically facilitate secure messagetransmission between non-mobile computing devices (e.g. computers 262 a,262 b of FIG. 4; remote desktop devices). Referring again to FIG. 4, inorder that signed messages received from senders may be read from mobiledevice 100 and encrypted messages be sent to those senders, mobiledevice 100 is adapted to store certificates and associated public keysof other individuals. Certificates stored on a user's computer 262 awill typically be downloaded from computer 262 a to mobile device 100through cradle 264, for example.

Certificates stored on computer 262 a and downloaded to mobile device100 are not limited to certificates associated with individuals but mayalso include certificates issued to CAs, for example. Certaincertificates stored in computer 262 a and/or mobile device 100 can alsobe explicitly designated as “trusted” by the user. Accordingly, when acertificate is received by a user on mobile device 100, it can beverified on mobile device 100 by matching the certificate with onestored on mobile device 100 and designated as trusted, or otherwisedetermined to be chained to a trusted certificate.

Mobile device 100 may also be adapted to store the private key of thepublic key/private key pair associated with the user, so that the userof mobile device 100 can sign outgoing messages composed on mobiledevice 100, and decrypt messages sent to the user encrypted with theuser's public key. The private key may be downloaded to mobile device100 from the user's computer 262 a through cradle 264, for example. Theprivate key is preferably exchanged between the computer 262 a andmobile device 100 so that the user may share one identity and one methodfor accessing messages.

User computers 262 a, 262 b can obtain certificates from a number ofsources, for storage on computers 262 a, 262 b and/or mobile devices(e.g. mobile device 100). These certificate sources may be private (e.g.dedicated for use within an organization) or public, may reside locallyor remotely, and may be accessible from within an organization's privatenetwork or through the Internet, for example. In the example shown inFIG. 4, multiple PKI servers 280 associated with the organization resideon LAN 250. PKI servers 280 include a CA server 282 for issuingcertificates, an LDAP server 284 used to search for and downloadcertificates (e.g. for individuals within the organization), and an OCSPserver 286 used to verify the revocation status of certificates.

Certificates may be retrieved from LDAP server 284 by a user computer262 a, for example, to be downloaded to mobile device 100 via cradle264. However, in a variant implementation, LDAP server 284 may beaccessed directly (i.e. “over the air” in this context) by mobile device100, and mobile device 100 may search for and retrieve individualcertificates through a mobile data server 288. Similarly, mobile dataserver 288 may be adapted to allow mobile device 100 to directly queryOCSP server 286 to verify the revocation status of certificates.

In variant implementations, only selected PKI servers 280 may be madeaccessible to mobile devices (e.g. allowing certificates to bedownloaded only from a user's computer 262 a, 262 b, while allowing therevocation status of certificates to be checked from mobile device 100).

In variant implementations, certain PKI servers 280 may be madeaccessible only to mobile devices registered to particular users, asspecified by an IT administrator, possibly in accordance with an ITpolicy, for example.

Other sources of certificates [not shown] may include a Windowscertificate store, another secure certificate store on or outside LAN250, and smart cards, for example.

Referring now to FIG. 6, a block diagram illustrating components of anexample of an encoded message, as may be received by a message server(e.g. message server 268 of FIG. 4), is shown generally as 350. Encodedmessage 350 typically includes one or more of the following: a headerportion 352, an encoded body portion 354, optionally one or more encodedattachments 356, one or more encrypted session keys 358, and signatureand signature-related information 360. For example, header portion 352typically includes addressing information such as “To”, “From”, and “CC”addresses, and may also include message length indicators, and senderencryption and signature scheme identifiers, for example. Actual messagecontent normally includes a message body or data portion 354 andpossibly one or more attachments 356, which may be encrypted by thesender using a session key. If a session key was used, it is typicallyencrypted for each intended recipient using the respective public keyfor each recipient, and included in the message at 358. If the messagewas signed, a signature and signature-related information 360 are alsoincluded. This may include the sender's certificate, for example.

The format for an encoded message as shown in FIG. 6 is provided by wayof example only, and persons skilled in the art will understand thatencoded messages may exist in other formats. For example, depending onthe specific messaging scheme used, components of an encoded message mayappear in a different order than shown in FIG. 6, and an encoded messagemay include fewer, additional, or different components, which may dependon whether the encoded message is encrypted, signed or both.

Embodiments of the invention are generally directed to a system andmethod that facilitates more efficient verification of digitalsignatures on certificates by storing certain information employed insignature verification operations for reuse. In building certificatechains (as discussed in the example of FIG. 5), the digital signatureson the certificates often need to be verified. Where multiplecertificates are processed on a user's computing device, the samedigital signature is often subject to verification more than once. Thismay be particularly prevalent where certificate chains containingcross-certificates are formed. Cross-certificates are discussed infurther detail below with reference to FIG. 7B.

Referring first to FIG. 7A, a block diagram showing two examplecertificate chains is shown. The two example certificate chains areillustrated generally as 400 a and 400 b. It will be understood bypersons skilled in the art that certificate chains 400 a and 400 b areprovided as examples. In particular, a certificate chain may comprise afewer or a greater number of certificates than depicted in the examplesshown.

Many organizations establish their own CAs, which issue certificatesspecifically to individuals within their own organizations. End entitycertificates issued to individuals within a particular organization neednot be issued by a single CA associated with the organization. An endentity certificate is often issued by one of a number of subordinate orintermediate CAs within a CA hierarchy headed by a root CA for theorganization. This root CA may provide a self-signed root certificate tobe used as a “trust anchor”—a starting point for the validation ofcertificates issued within the organization.

Certificate chain 400 a depicts an example chain of certificates formedto validate a certificate 402 a issued to “user1”, an individual withinorganization “ABC”. Certificate 402 a chains to a self-signed rootcertificate 404 a, issued by a root CA of the organization and trustedby user1, via an intermediate certificate 406 a issued by the root CA toan intermediate CA of the organization. The certificates issued withinorganization ABC may be searched and retrieved from an LDAP servermaintained by the organization (e.g. LDAP server 284 of FIG. 4), forexample.

Similarly, certificate chain 400 b depicts an example chain ofcertificates formed to validate a certificate 402 b issued to “user2”,an individual within a different organization “XYZ”. Certificate 402 bchains to a self-signed root certificate 404 b issued by a root CA oforganization XYZ and trusted by user2, via an intermediate certificate406 b. The certificates issued within organization XYZ may be searchedand retrieved from an LDAP server maintained by organization XYZ, forexample.

Consider an example situation where user1 of organization ABC receivesan encoded message from user2 of organization XYZ. Even if user2 hasattached his certificate 402 b to the message, user1 will be unable toverify the trust status of user2's certificate 402 b with thatcertificate alone (assuming that user1 has not already stored user2'scertificate 402 b and marked it as trusted). If user1 does not trustcertificates from organization XYZ, then user2's certificate 402 bcannot be validated since it does not chain to a trusted certificate.

In order to facilitate secure communications between users of differentorganizations, it may be desirable to allow certificates to be used andtrusted between the organizations. An authentication method known ascross-certification may be performed between two organizations, where aCA of one organization certifies a CA of the other organization.

The term cross-certification may be used to refer generally to twooperations. The first operation, which is typically executed relativelyinfrequently, relates to the establishment of a trust relationshipbetween two CAs (e.g. across organizations or within the sameorganization), through the signing of one CA's public key by another CA,in a certificate referred to as a cross-certificate. The secondoperation, which is typically executed relatively frequently, involvesverifying a user's certificate through the formation of a certificatechain that includes at least one such cross-certificate.

Referring now to FIG. 7B, a block diagram showing examples ofcross-certificates linking two example certificate chains is shown. Across-certificate 410 issued to the root CA of organization ABC by theroot CA of organization XYZ is shown in this example. Similarly, across-certificate 412 issued to the root CA of organization XYZ by theroot CA of organization ABC is shown.

The example of FIG. 7B illustrates mutual cross-certification betweentwo root CAs. However, other cross-certification methods are possible invariant implementations. For example, cross-certificates may be issuedby a subordinate CA in one organization to the root CA of anotherorganization. As a further example, a CA of a first organization mayissue a cross-certificate to a CA of a second organization, even if across-certificate is not issued back to the first organization by thesecond organization.

Furthermore, certificate usage across organizations may be restricted,as dictated by an organization's IT policy, for example. For instance,the IT policy of one organization may dictate that certificates fromother organizations will be trusted only for the purpose of processingencoded e-mail messages. Also, cross-certificates may be revoked by anissuing CA of one organization to terminate trust relationships withother organizations. This can facilitate more efficient control ofsecure e-mail communications between individuals across differentorganizations.

Cross-certificates facilitate secure communications between individualsof organizations that have established a trust relationship. Consideragain the situation where user1 of organization ABC receives an encodedmessage from user2 of organization XYZ. User1 will be able to verify thetrust status of user2's certificate 402 b, by retrieving certificates ina chain from user2's certificate 402 b, to root certificate 404 a issuedby a root CA of user1's organization and trusted by user1. Specifically,as shown in the example of FIG. 7B, the chain includes ABC's rootcertificate 404 a, cross-certificate 412, XYZ's root certificate 404 b,intermediate certificate 406 b, and user2's certificate 402 b.

For user1 to verify the trust status of user2's certificate 402 b, user1must obtain certificate 402 b. This will customarily accompany themessage from user2 to user1; however, in the event that certificate 402b is not provided and is not otherwise stored on user1's computingdevice, it must be retrieved, from an LDAP server maintained byorganization XYZ, or other certificate server, for example. Furthermore,each of the remaining certificates in the chain must also be retrievedto verify the trust status of certificate 402 b. The other certificatesin the chain, which in this example include a root certificate and across-certificate, would need to be retrieved from ABC's LDAP server,XYZ's LDAP server, or some other LDAP server accessible to user1.

As discussed with reference to FIG. 5, and FIGS. 7A and 7B, the digitalsignatures of issuing CAs on certificates often need to be verified whenbuilding certificate chains. Other tasks may also be performed whenvalidating certificates, such as checking the validity of acertificate's date, or checking other validation criteria that might beestablished by an organization in accordance with an IT policy, forexample.

Verification of a digital signature on a certificate is a process thatrequires the public key of the issuing CA. When a CA digitally signs acertificate, certificate information including the name and public keyof the certificate holder for example, or a hash of that informationobtained through application of a hashing algorithm, is typicallyencoded using the CA's private key. The algorithm used by the issuing CAto sign a certificate is typically identified in the certificate.Subsequently, in a manner similar to that employed in verifying thedigital signature of a message signed by a user, the CA's digitalsignature on a certificate can be verified by decoding the encodedinformation or hash using the CA's public key, and comparing the resultto the expected certificate information or hash thereof respectively. Asuccessful match indicates that the CA has verified that the certificateholder's public key may be validly bound to the certificate holder, andsuggests that the certificate holders public key can be trusted if theCA is trusted.

Verifying certificate signatures can be a process that is bothtime-consuming and costly (e.g. in terms of computing resource usage),particularly where the verifications are performed on small devices,such as mobile devices for example. Embodiments of the invention aregenerally directed to a system and method that facilitates moreefficient verification of digital signatures on certificates by storingcertain information employed in signature verification operations forreuse.

In at least one embodiment, one or more public keys of a CA that hasissued a particular certificate are associated with that certificate,and cached or stored. As indicated above, when attempting to verify adigital signature on a certificate signed by a CA, the CA's public keyis required. However, there may exist multiple certificates (each with apublic key attached) that appear to belong to the same CA. Thissituation might arise if several certificates have the same or similarsubject data (i.e. the certificate data which identifies the certificateholder) or if the CA has been issued multiple public keys (some of whichmay no longer be valid), for example. Accordingly, it can be beneficialto track which particular public key has been used to successfullyverify a particular certificate.

Referring to FIG. 8, a flowchart illustrating steps in a method ofverifying digital signatures on certificates in an embodiment of theinvention is shown generally as 420.

In one embodiment of the invention, at least some of the steps of themethod are performed by a certificate validation application thatexecutes and resides on a mobile device. In variant embodiments, thecertificate validation application may be residing and executing on acomputing device other than a mobile device. Furthermore, thecertificate validation application need not be a stand-aloneapplication, and the functionality of the certificate validationapplication may be implemented in one or more applications executing andresiding on the mobile or other computing device.

Generally, in method 420, when a given public key is used insuccessfully verifying the digital signature on a certificate, a copy ofthat public key is cached, or otherwise stored in a memory store. Forexample, the public key may be stored with the certificate dataassociated with the certificate, or in a separate memory store (e.g. alookup table) adapted to store public keys employed in successfulsignature verifications. When a subsequent attempt to verify the digitalsignature on the same certificate is made, rather than immediatelyperforming an expensive signature verification operation requiring atleast the decoding of some data using a public key, the public key thatwould have been used to verify the digital signature again is insteadinitially compared to the stored public key. If these public keys match,then the verification will be deemed successful, since the public key tobe used matches a key that has been previously used successfully in asignature verification operation. It is considered unnecessary toperform an actual signature verification operation again for the samedigital signature. Accordingly, at least some subsequent signatureverification operations may be replaced by more efficient (e.g. bytearray) comparison operations. The steps of method 420 are described infurther detail below.

At step 430, a verification of a digital signature on a certificate isinitiated (e.g. by the certificate validation application).Verifications of digital signatures on certificates may be performed,for instance, when building certificate chains in order to validatespecific certificates received by a user (e.g. to verify the truststatus of a certificate attached to a received message as discussed withreference to FIG. 5). In this embodiment, the digital signatures on thecertificates being verified are those of the certification authoritiesthat issued the respective certificates. As noted earlier, in asignature verification operation, a public key of the certificationauthority that issued the certificate is required. Certificate(s) andpublic key(s) of the certification authority may need to be retrieved atthis step (e.g. from an LDAP server) if they are not already stored in acertificate store on the mobile or other computing device.

For a given public key, at step 440, prior to performing the signatureverification operation using this public key, a determination is made asto whether the digital signature on the subject certificate haspreviously been successfully verified using this public key. Asindicated above, this may be done by comparing a stored public key forthe certificate issuer previously used to successfully verify thedigital signature on the subject certificate (if one exists, as storedat step 470 in the cache or other memory store) with the public key thatis about to be used to verify the digital signature, and thendetermining if there is a match. Since only public keys employed insuccessful verification attempts are stored in the cache or other memorystore in this embodiment, if a match were determined, this would suggestthat the digital signature on the subject certificate has previouslybeen successfully verified.

If the digital signature on the subject certificate has not beenpreviously successfully verified using the given public key, then atstep 450, the digital signature is verified using this public key inknown manner. If the signature is successfully verified as determined atstep 460 using this public key, then the public key used in thissuccessful verification is stored in the cache or other memory store forfuture use at step 470, in accordance with this embodiment. For example,the public key stored at step 470 may be stored with the data associatedwith the subject certificate, or in a central memory store for publickeys (e.g. in a lookup table) indexed by certificate (e.g. by storingthe issuer name and serial number of the certificate with the publickey).

On the other hand, if the digital signature on the subject certificatehad previously been successfully verified using the given public key asdetermined at step 440, then at step 480, an indication that theverification is successful is provided. This is done in lieu ofperforming an actual signature verification operation requiring at leastthe decoding of some data using a public key, thereby making thesignature verification process more efficient. This may help conservebattery power and enhance the user experience, for example, particularlyfor small devices such as mobile devices.

The steps of method 420 may be repeated for additional public keys.

Referring now to FIG. 8B, a flowchart illustrating steps in a method ofverifying digital signatures on certificates in another embodiment ofthe invention is shown generally as 420 b.

Method 420 b is similar to method 420, except that in contrast to method420 where only the public keys employed in successful signatureverifications are stored in the cache or other memory store, in method420 b, the public keys used in any signature verification attempt(whether successful or unsuccessful) are stored in the cache or othermemory store along with the result of the verification attempt.

Generally, in method 420 b, when a given public key is used in verifyingthe digital signature on a certificate, a copy of that public key iscached or otherwise stored in a memory store, along with the result ofthe operation. For example, the public key and associated result may bestored with the certificate data associated with the certificate, or ina separate memory store (e.g. a lookup table). When a subsequent attemptto verify the digital signature on the same certificate is made usingthe given public key, rather than performing an expensive signatureverification operation requiring at least the decoding of some datausing that public key, the public key that would have been used toverify the digital signature again is instead initially compared to thestored public key(s). If the given public key matches a stored publickey, then the current verification attempt will be deemed successful ornot successful, depending on the stored result associated with thatstored public key. If the stored result indicates that the previousverification attempt with that stored public key was successful, thenthe current verification attempt will be deemed to succeed. If thestored result indicates that the previous verification attempt with thatstored public key was not successful, then the current verificationattempt will be deemed to fail. Accordingly, subsequent signatureverification operations that would otherwise require decoding of somedata using public keys may be replaced by more efficient (e.g. bytearray) comparison operations.

At step 430, a verification of a digital signature on a certificate isinitiated (e.g. by the certificate validation application), as describedwith reference to method 420.

For a given public key, at step 440 b, prior to performing the signatureverification operation using this public key, a determination is made asto whether the digital signature on the subject certificate haspreviously been verified using this public key. As indicated above, thismay be done by comparing a public key for the certificate issuerpreviously used to verify the digital signature on the subjectcertificate (if one exists, as stored at step 470 in the cache or othermemory store) with the public key that is about to be used to verify thedigital signature, and determining if there is a match. If a match weredetermined, this would suggest that an attempt to verify the digitalsignature on the subject certificate was previously made.

If an attempt to verify the digital signature on the subject certificatewas not previously made, then a signature verification operation isperformed in known manner at step 450, as similarly described withreference to method 420. Both the public key used in the verificationand the result of the verification attempt (i.e. an indicator of whetherthe digital signature was successfully or unsuccessfully verified) arestored in the cache or other memory store for future use at step 470 b,in accordance with this embodiment. For example, the public key andresult stored at step 470 b may be stored with the data associated withthe subject certificate, or in a central memory store for public keys(e.g. in a lookup table) indexed by certificate (e.g. by storing theserial number of the certificate with the public key).

If the digital signature on the subject certificate has previously beenverified with the given public key as determined at step 440 b, then atstep 472, the result of the previous verification attempt with this keyis retrieved from the cache or other memory store and a determination ismade as to whether or not the stored result indicates that the previousverification attempt with this key was successful. If so, then at step480, an indication that the current verification is to succeed isprovided; if not, then at step 490, an indication that the currentverification is not to succeed is provided.

The steps of method 420 b may be repeated for additional public keys.

In lieu of performing a signature verification operation requiring atleast the decoding of some data using a given public key, the results ofprevious verification attempts are used to determine if a verificationusing this public key should fail, thereby making the signatureverification process more efficient. In particular, if a user requestsverification of the digital signature of a certificate multiple timesusing the same invalid public key, then an actual expensive signatureverification operation requiring at least the decoding of some datausing the public key need be performed only once, and the subsequentattempts will fail immediately after performing a relatively efficient(e.g. byte array) comparison operation. This may further help conservebattery power and enhance the user experience, for example, particularlyfor small devices such as mobile devices.

It will be understood by persons skilled in the art that otherinformation in addition to the public keys and verification attemptresults described above may also be stored in the cache or other memorystore, if desired, in variant embodiments.

In a variant embodiment of the invention, public keys and otherinformation (e.g. verification attempt results) stored in the cache orother memory store may only be permitted for use in public keycomparisons for a limited duration, after which they may be consideredstale and subject to deletion from the cache or other memory store. Thismay be done for security purposes so that an actual signatureverification operation requiring at least the decoding of some datausing a public key must be re-performed from time-to-time. This durationmay be set in accordance with IT Policy, for example. Similarly, inanother variant embodiment of the invention, some or all of the publickeys and other information stored in the cache or other memory store maybe marked as stale or deleted as may be directed manually by a user oradministrator, for example, so that the signature verification operationmust be re-performed. For more enhanced security, validation operationsmay also be performed to ensure that public keys (e.g. public keys whichpreviously successfully verified a certificate signature) have notbecome invalid after storage, for example.

The steps of a method of verifying digital signatures on certificates inembodiments of the invention may be provided as executable softwareinstructions stored on computer-readable media, which may includetransmission-type media.

The invention has been described with regard to a number of embodiments.However, it will be understood by persons skilled in the art that othervariants and modifications may be made without departing from the scopeof the invention as defined in the claims appended hereto.

The invention claimed is:
 1. A non-transitory computer readable medium storing instructions executable by a processor of a computing device, which, when executed by the processor, cause the processor to: store, in a memory store, a stored public key in response to a first successful verification of a digital signature on a certificate on the computing device, the first successful verification resulting from performing a signature verification operation on the digital signature using the stored public key; receive a public key associated with an issuer of the certificate, and a request to verify the digital signature of the certificate using the received public key; compare the received public key to the stored public key, in lieu of performing the signature verification operation on the digital signature using the received public key; and indicate a second successful verification of the digital signature in response to determining that the received public key matches the stored public key.
 2. The non-transitory computer readable medium of claim 1, wherein the computing device comprises a mobile device.
 3. The non-transitory computer readable medium of claim 1, wherein the instructions, when executed by the processor, further cause the processor to identify the stored public key as stale if the stored public key has been stored in the memory store for longer than a pre-determined duration.
 4. The computer readable medium of claim 3, wherein the instructions, when executed by the processor, further cause the processor to compare the received public key with the stored public key residing in the memory store.
 5. The non-transitory computer readable medium of claim 4, wherein the instructions, when executed by the processor, further cause the processor to indicate successful verification of the digital signature if the stored public key is not identified as stale.
 6. The non-transitory computer readable medium of claim 3, wherein the instructions, when executed by the processor, further cause the processor to delete the stored public key from the memory store if the stored public key is identified as stale.
 7. The non-transitory computer readable medium of claim 1, wherein the instructions, when executed by the processor, further cause the processor to mark the stored public key as stale in response to a user request.
 8. The non-transitory computer readable medium of claim 7, wherein the instructions, when executed by the processor, further cause the processor to compare the received public key with the stored public key residing in the memory store.
 9. The non-transitory computer readable medium of claim 8, wherein the instructions, when executed by the processor, further cause the processor to indicate successful verification of the digital signature if the stored public key is not marked as stale.
 10. The non-transitory computer readable medium of claim 7, wherein the instructions, when executed by the processor, further cause the processor to delete the stored public key from the memory store if the stored public key is marked as stale.
 11. A non-transitory computer readable medium storing instructions executable by a processor of a computing device, which, when executed by the processor, cause the processor to: store, in a memory store, a stored public key and a prior verification result in response to a first successful verification of a digital signature on a certificate on the computing device, the first successful verification resulting from performing a signature verification operation on the digital signature using the stored public key; receive a public key associated with an issuer of the certificate, and a request to verify the digital signature of the certificate using the received public key; compare the received public key to the stored public key, in lieu of performing the signature verification operation on the digital signature using the received public key; indicate a second successful verification of the digital signature in response to determining that the received public key matches the stored public key, and that the prior verification result associated with the stored public key was successful; and indicate unsuccessful verification of the digital signature in response to determining that the received public key matches the stored public key, and that the prior verification result was unsuccessful.
 12. The non-transitory computer readable medium of claim 11, wherein the computing device comprises a mobile device.
 13. The non-transitory computer readable medium of claim 11, wherein the instructions, when executed by the processor, further cause the processor to identify the stored public key as stale if the stored public key has been stored in the memory store for longer than a pre-determined duration.
 14. The non-transitory computer readable medium of claim 13, wherein the instructions, when executed by the processor, further cause the processor to compare the received public key with the stored public key residing in the memory store.
 15. The non-transitory computer readable medium of claim 14, wherein the instructions, when executed by the processor, further cause the processor to indicate successful or unsuccessful verification of the digital signature depending on the prior verification result if the stored public key is not identified as stale.
 16. The non-transitory computer readable medium of claim 13, wherein the instructions, when executed by the processor, further cause the processor to delete the stored public key from the memory store if the stored public key is identified as stale.
 17. The non-transitory computer readable medium of claim 11, wherein the instructions, when executed by the processor, further cause the processor to mark the stored public key as stale in response to a user request.
 18. The non-transitory computer readable medium of claim 17, wherein the instructions, when executed by the processor, further cause the processor compare the received public key with the stored public key residing in the memory store.
 19. The non-transitory computer readable medium of claim 18, wherein the instructions, when executed by the processor, further cause the processor to indicate successful or unsuccessful verification of the digital signature depending on the prior verification result if the stored public key is not marked as stale.
 20. The non-transitory computer readable medium of claim 17, wherein the instructions, when executed by the processor, further cause the processor to delete the stored public key from the memory store if the stored public key is marked as stale. 